Folate mimetics and folate-receptor binding conjugates thereof

ABSTRACT

A cell population expressing folate receptors is selectively targeted with a non-peptide folic acid analogue. The non-peptide folic acid analogue is conjugated to a diagnostic or therapeutic agent to enable selective delivery of the agent to the targeted cell population.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/286,082, filed Apr. 24, 2001, which is incorporated herein byreference in its entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Grant R01-CA70845awarded by the National Institutes of Health—National Cancer Institute.The Government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to folate mimetics and their use in therapeuticand diagnostic applications. More particularly, this invention relatesto using des-glutamyl folic acid analogs recognized by and selectivelybound by folate receptors and other folate binding proteins and the useof such analogs for targeted delivery of diagnostic or therapeuticagents to folate-receptor bearing cell populations.

BACKGROUND OF THE INVENTION

A number of methods are known for selectively targeting cells in apatient for delivery of diagnostic or therapeutic agents. Selectivetargeting has led to the introduction of various diagnostic agents forvisualization of tissues, such as contrast agents useful in MagneticResonance Imaging (MRI), radiodiagnostic compositions, and the like.Introduction of therapeutic agents, such as compositions forradiotherapy or for neutron capture therapy, compositions forchemotherapy, various proteins, peptides, and nucleic acids, proteintoxins, antisense oligonucleotides, liposomes, analgesics, antibiotics,antihypertensive agents, antiviral agents, antihistamines, expectorants,vitamins, plasmids, and the like, has also been demonstrated.

Folate conjugates have been used for the selective targeting of cellpopulations expressing folate receptors or other folate binding proteinsto label or deliver bioactive compounds to such cells. The relativepopulations of these receptors and binding proteins have been exploitedin achieving selectivity in the targeting of certain cells and tissues,such as the selective targeting of tumors expressing elevated levels ofhigh-affinity folate receptors. The following publications, thedisclosures of which are incorporated herein by reference, illustratethe nature and use of folate conjugates for diagnosis or delivery ofbiologically significant compounds to selected cell populations inpatients in need of such diagnosis or treatment:

-   (a) Leamon and Low, “Cytotoxicity of Momordin-folate Conjugates in    Cultured Human Cells” in J. Biol. Chem., 1992, 267, 24966-24967.-   (b) Leamon et al., “Cytotoxicity of Folate-pseudomonas Exotoxin    Conjugates Towards Tumor Cells” in J. Biol. Chem., 1993, 268,    24847-24854.-   (c) Lee and Low, “Delivery of Liposomes into Cultured Kb Cells via    Folate Receptor-mediated Endocytosis” in J. Biol. Chem., 1994, 269,    3198-3204.-   (d) Wang et al., “Delivery of Antisense Oligonucleotides Against the    Human Epidermal Growth Factor Receptor into Cultured Kb Cells with    Liposomes Conjugated to Folate via Polyethyleneglycol” in Proc.    Natl. Acad. Sci. U.S.A., 1995, 92, 3318-3322.-   (e) Wang et al., “Synthesis, Purification and Tumor Cell Uptake of    Ga-67-deferoxamine-folate, a Potential Radiopharmaceutical for Tumor    Imaging” in Bioconj. Chem., 1996, 7, 56-63.-   (f) Leamon et al., “Delivery of Macromolecules into Living Cells: a    Method That Exploits Folate Receptor Endocytosis” in Proc. Natl.    Acad. Sci., U.S.A., 1991, 88, 5572-5576.-   (g) Krantz et al., “Conjugates of Folate Anti-Effector Cell    Antibodies” in U.S. Pat. No. 5,547,668.-   (h) Wedeking et al., “Metal Complexes Derivatized with Folate for    Use in Diagnostic and Therapeutic Applications” in U.S. Pat. No.    6,093,382.-   (i) Low et al., “Method for Enhancing Transmembrane Transport of    Exogenous Molecules” in U.S. Pat. No. 5,416,016.-   (j) Miotti et al., “Characterization of Human Ovarian    Carcinoma-Associated Antigens Defined by Novel Monoclonal Antibodies    with Tumor-Restricted Specificity” in Int. J. Cancer, 1987, 39,    297-303.-   (k) Campell et al., “Folate-Binding Protein is a Marker for Ovarian    Cancer” in Cancer Res., 1991, 51, 5329-5338.-   (l) Jansen et al., “Identification of a Membrane-Associated    Folate-Binding Protein in Human Leukemic CCRF-CEM Cells with    Transport-Related Methotrexate Resistance” in Cancer Res., 1989, 49,    2455-2459.

Multiple types of folate recognition sites present on cells, such asα-folate receptors, β-folate receptors, folate binding proteins, and thelike, have been shown to recognize and bind the conjugates describedabove. The primary pathway for entry of folate derivatives into cells isthrough a facilitated transport mechanism mediated by a membranetransport protein. However, when folate is covalently conjugated tocertain small molecules and macromolecules, the transport system canfail to recognize the folate molecule.

Advantageously, in addition to the facilitated transport protein, somecells possess a second membrane-bound receptor, folate binding protein(FBP), that allows folate uptake via receptor-mediated endocytosis. Atphysiological plasma concentrations (nanomolar range), folic acid bindsto cell surface receptors and is internalized via an endocytic process.Receptor-mediated endocytosis is the movement of extracellular ligandsbound to cell surface receptors into the interior of the cells throughinvagination of the membrane, a process that is initiated by the bindingof a ligand to its specific receptor. The uptake of substances byreceptor-mediated endocytosis is a characteristic ability of somenormal, healthy cells such as macrophages, hepatocytes, fibroblasts,reticulocytes, and the like, as well as abnormal or pathogenic cells,such as tumor cells. Notably, folate binding proteins involved inendocytosis are less sensitive to modification of the folate moleculethan the membrane transport proteins, and often recognize folateconjugates. Both targeting and uptake of conjugated diagnostic andtherapeutic agents are enhanced.

Following endosome acidification, the folate receptor changesconformation near its ligand-binding domain and releases the folic acidmolecule. Folate receptors are known to recycle back to the membranesurface for additional rounds of ligand-mediated internalization.However, a significant fraction of the internalized receptor-folic acidcomplex has been shown to return back to the cell surface shortly afterendocytosis. This suggests that the acid-triggered ligand releasemechanism does not proceed to completion, at least after the first roundof internalization (Kamen et al., 1988, J. Biol. Chem. 263,13602-13609).

Pteroic acid, which is essentially folic acid lacking the distalglutamyl residue (FIG. 1), does not bind to the high-affinity folatereceptor to any appreciable extent (Kamen et al., 1986, Proc. Natl.Acad. Sci., U.S.A. 83, 5983-5987); in fact, 2 μM pteroic acid (100-foldexcess) had absolutely no effect on the binding of folate to the folatereceptor. Thus, the glutamyl residue of folic acid, or some portionthereof, was generally thought to be required for efficient, specificreceptor recognition. However, recent studies have revealed that theglutamyl residue of folic acid could be replaced with a lysyl residuewithout disturbing the binding affinity of the ligand (McAlinden et al.,1991, Biochemistry 30, 5674-5681.; Wu et al., 1997, J. Membrane Biol.159, 137-147), that the glutamyl residue can be replaced with a glycylresidue without substantially altering cellular uptake, and that noselective isomeric (i.e., α-glutamyl vs. γ-glutamyl) conjugationrequirement necessarily exists (Leamon et al., J. Drug Targeting7:157-169 (1999); Linder et al., J. Nuclear Med. 41(5):470 Suppl. 2000).

Efforts to improve the selectivity of targeting or increase thediversity of the agents delivered to the cell or tissue have beenhampered by a number of complications, including the complex synthesesrequired for the preparation of these conjugates. Such synthetic schemesare not only time consuming, but may also preclude the use of certainconjugates due to synthetic incompatibilities. A folic acid analogcapable of expanding the number or diversity of agents, via theconjugates of such agents and these folic acid analogs, presentable totarget cells would be advantageous.

SUMMARY OF THE INVENTION

The present invention provides a compound that is capable of binding asa ligand to a folate recognition site. The compound is referred to as a“non-peptide folic acid analog.”

The present invention also provides a ligand-agent conjugate capable ofbinding to a folate recognition site, the ligand-agent conjugatecomprising a diagnostic or therapeutic agent in association with anon-peptide folic acid analog.

The present invention also provides a ligand-agent conjugate capable ofbinding to a folate recognition site with high affinity, theligand-agent conjugate comprising a diagnostic or therapeutic agent inassociation with a plurality of non-peptide folic acid analogs.

The present invention also provides a method for targeting a cell ortissue with a diagnostic or therapeutic agent, comprising the step ofadministering to a patient an effective amount of a ligand-agentconjugate comprising a diagnostic or therapeutic agent in associationwith a non-peptide folic acid analog.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of folic acid and pteroic acid.

FIG. 2 is a schematic representation of the synthesis of a pteroic acidconjugate, CYK4-013.

FIG. 3 is a schematic representation of the synthesis of pteroic acidconjugate linked to the tetraazamacrocyclic DOTA chelating ligand(CY4-036).

FIG. 4 is a schematic representation of the metabolism of a ligand-agentconjugate of the invention involving bioreduction to release the agent.

FIG. 5 is a schematic representation of the metabolism of a ligand-agentconjugate of the invention involving acid hydrolysis to release theagent.

FIG. 6 depicts the binding activity of (S)-α-carboxybenzoyl pteroate(ACBP) and N-pteroyl-2-amino-2-carboxymethylpyridine (Pte-AP).

FIG. 7 is a schematic representation of the synthesis ofpteroylhydrazido-benzenetetracarboxylic acid-diacetoxyscirpenol(Pte-hydrazideo-BTCA-DAS).

FIG. 8 depicts the binding activity of pteroyl hydrazide (Pte-hydrazide)and Pte-hydrazido-BTCA-DAS.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a ligand capable of binding to a folaterecognition site, comprising a non-peptide folic acid analog of generalformula

wherein

X and Y are each independently selected from the group consisting ofhalo, R², OR², SR³, and NR⁴R⁵;

U, V, and W represent divalent moieties each independently selected fromthe group consisting of —(R^(6′))C═, —N═, —(R^(6′))C(R^(7′))—, and—N(R^(4′))—;

T is selected from the group consisting of S, O, N and —C═C— such thatthe ring structure of which T is a member is aromatic;

A¹ and A² are each independently selected from the group consisting of—C(Z)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z)—, —O—C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur;

R¹ is selected from the group consisting of hydrogen, halo, C₁-C₁₂alkyl, and C₁-C₁₂ alkoxy;

R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″), R^(6″) and R^(7″) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂ alkenyl, C₁-C₁₂alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂ alkylamino)carbonyl;

R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or, R⁶ and R⁷ are takentogether to form O═;

R^(6′) and R^(7′) are each independently selected from the groupconsisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or,R^(6′) and R^(7′) are taken together to form O═;

L is a divalent linker;

n, p, r and s are each independently either 0 or 1; and

B is hydrogen or a leaving group;

provided that the linker L does not include a naturally occurring aminoacid covalently linked to A² at its α-amino group through an amide bond.It should be understood that the structure of formula I includestautomeric structures, for example in compounds where X is OH, SH or NH.

In the compound of the invention wherein any one or more of A¹, A², R¹,R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″), R^(6″), R^(7″), R⁶, R⁷, R^(6′)and R^(7′) comprises an alkyl, alkoxy, alkylamino, alkanoyl, alkenyl,alkynyl, alkoxy carbonyl, or alkylamino carbonyl group, the grouppreferably contains 1 to 6 carbon atoms (C₁-C₆); more preferably itcontains 1 to 4 carbon atoms (C₁-C₄).

Folic acid contains a glutamyl residue bound at its α-amino group via anamide bond to the benzoate moiety of pteroic acid (FIG. 1). This amidebond would typically not be classified as a “peptide” bond becausepteroic acid is not an amino acid; a peptide bond is typicallycharacterized as a bond in which the carboxyl group of one amino acid iscondensed with the amino group of another to form a —CO.NH— linkage.

Nonetheless, for ease of reference, the compound of formula I, which isdefined as having linker L that lacks a glutamyl or any other naturallyoccurring amino acid residue covalently linked to A² at its α-aminogroup through an amide bond, is termed herein a “non-peptide” folic acidanalog. That is, the term “non-peptide” as used herein in reference tothe compound of formula I, means that linker L does not include anaturally occurring amino acid covalently linked to A² through an amidebond at its α-amino group, thereby distinguishing the compound offormula I from, for example, folic acid, pteroyl-γ-glutamate-cysteine,pteroyl-α-glutamate-cysteine, and pteroyl-glycine-cysteine (Leamon etal., J. Drug Targeting 7:157-169 (1999)). In a preferred embodiment ofthe non-peptide folic acid analog of the invention, linker L does notinclude any amino acid (whether naturally occurring or non-naturallyoccurring) covalently linked to A² through an amide bond at its α-aminogroup.

It should be further understood that the compound of formula I cancontain a naturally occurring amino acid covalently linked to A² at asite other than its α-amino group, a non-naturally occurring amino acidcovalently linked to A² through an amide bond or otherwise, as well asany other non-amino acid moiety covalently linked to A² through an amidebond or otherwise, as defined with reference to the formula.

The general chemical terms used in the formulae above have their usualmeanings. For example, the term “alkyl” as used herein refers to alinear or branched chain of carbon atoms, such as methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl,2-pentyl, 3-pentyl, neopentyl, hexyl, heptyl, octyl and the like.

The term “alkoxy” as used herein refers to alkyl, as defined above,substituted with oxygen, such as methoxy, ethoxy, propoxy, isopropoxy,butoxy, tert-butoxy and the like.

The term “alkanoyl” as used herein refers to formyl, or alkyl, asdefined above, terminally-substituted with a carbonyl such as acetyl,propanoyl, butanoyl, pentanoyl and the like.

The term “alkenyl” as used herein refers to a linear or branched chainof carbon atoms with one or more carbon-carbon double bonds, such asvinyl.

The term “alkynyl” as used herein refers to a linear or branched chainof carbon atoms with one or more carbon-carbon triple bonds.

The term “alkylamino” as used herein refers to alkyl, as defined above,substituted with nitrogen, including both monoalkylamino such asmethylamino, ethylamino, propylamino, tert-butylamino, and the like, anddialkylamino such as dimethylamino, diethylamino, methylpropylamino, andthe like.

The term “halo” as used herein refers to any Group 17 element andincludes fluoro, chloro, bromo, iodo, and astatine(o).

The term “alkylenyl” as used herein refers to a divalent linear orbranched chain of carbon atoms such as methylene, ethylene,2-methylpropylene, and the like.

The term “leaving group” as used herein refers to a functionality thatmay be replaced, such as an activated halo or alkoxy, by an introducedsubstituent, such as a alkylamino, carbon nucleophile, a differentalkoxy, a different halo, and the like.

The term “naturally occurring amino acid” as used herein refers to the20 coded amino acids available for endogenous protein synthesis, such asglycine, alanine, methionine, and the like.

A preferred embodiment of the ligand is one having the general formula Iwherein p is 1, s is 1, and T, U, V, W, X, Y, R¹, R⁶, R⁷ and A¹ areselected such that at least a portion of the molecule is isosteric withpteroic acid. By “isosteric” it is meant that the two compounds orportions of compounds comprise isosteric substituents that occupysimilar volumes and, preferably but not necessarily, have similarelectronic character. As a nonlimiting example, hydrogen, halo, CH₃, OH,SH and NH₂ may be considered for purposes of this invention as beingisosteric substituents.

Folate receptor activity is expected to be retained when isostericsubstitutions are made to that portion of the non-peptide folic acidanalog that is derived from pteroic acid. For example, as reported inJansen, “Receptor- and Carrier-Mediated Transport Systems for Folatesand Antifolates,” in Anticancer Drug Development Guide: Antifolate Drugsin Cancer Therapy, Jackman, Ed., Humana Press Inc, Totowa N.J. (1999),ring substituents X and Y, ring components U, V, W, T, and A¹ can besubstituted in the pteroic acid reference structure while in most casesretaining folate receptor affinity.

An example of a preferred ligand according to the invention having aportion that is isosteric with pteroic acid is a ligand having formula I(including tautomers thereof) wherein

X and Y are each independently selected from the group consisting ofhydrogen, halo, CH₃, OH, SH and NH₂, with X more preferably being OH;

U, V and W represent divalent moieties each independently selected fromthe group consisting of —CH═ and —N═;

A¹ is selected from the group consisting of —C(Z)—, —NH—, —N(CH₃)—, —O—,—S—, —S(O)—, —S(O)₂—, —CH₂—, —CH(CH₃)—, —C(CH₃)₂—, —N(CH₂—C≡CH)— and—N(C≡CH)—; where Z is oxygen or sulfur;

R¹ is selected from the group consisting of hydrogen, halo and methyl;

R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, CH₃, OH, SH and NH₂; or, R⁶ and R⁷ are taken together toform O═;

A² is selected from the group consisting of —C(Z)—, —C(Z)O—, —OC(Z)—,—N(R^(4″))—, —C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—, —O—C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z)—O—, —N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—,—S(O)₂—, —N(R^(4″))S(O)₂—, —C(R^(6″))(R^(7″))—, C₁-C₆ alkyl; C₁-C₆alkoxy; where Z is oxygen or sulfur;

p and rare each 1; and

T, R^(4″), R^(5″), R^(6″), R^(7″), L, n, s and B are as defined above;provided that the linker L does not include a naturally occurring aminoacid covalently linked to A² at its α-amino group through an amide bond.More preferably, T is —C═C—.

A particularly preferred ligand of the invention is a derivative ofpteroic acid and has formula I (including tautomers thereof) wherein Xis OH; Y is NH₂; U and W are each —N═; V is —CH═; T is —C═C—; A¹ is—NH—; R¹ is hydrogen; A² is —C(O)—, —C(O)O—, or —C(O)NH— and is para toA¹; R⁶ and R⁷ are hydrogen; p, r and s are each 1; and L, n and B are asdefined elsewhere herein; provided that the linker L does not include anaturally occurring amino acid covalently linked to A² at its α-aminogroup through an amide bond.

One embodiment of the invention is a ligand capable of binding to afolate recognition site, such as a folate binding protein, folatereceptor, and the like. Such a non-peptide folate analog may also bedescribed as a folate mimetic. Compounds illustrative of this embodimentare selected from the general formula I. Such analogs may operate assurrogates for folate in methods utilizing folate, such as targetingmolecules for cells or tissues expressing folate recognition sites.

In a particularly preferred embodiment, the compound of the inventionhas formula I and further exhibits binding affinity for a folatereceptor. A relative binding affinity assay is described in detail inExample IV and in Westerhoff et al. (Mol. Pharm., 1995, 48:459-471).Performing this assay is straightforward. A preferred compound exhibitsa binding affinity for the folate receptor relative to folic acid of atleast about 0.01, more preferably at least about 0.05, even morepreferably at least about 0.10, even more preferably at least about0.25, even more preferably at least about 0.50, and most preferably atleast about 0.75, wherein the binding affinity of folic acid for thefolate receptor is defined as 1.0. It should be understood that thebinding affinity of the compound of the invention may exceed 1.0, incases where the binding affinity of the compound for the folate receptoris greater than that of folic acid itself.

The compounds of formula I may optionally include a linker, spacer, orcouple of variable length. The linker, spacer, or couple, hereinaftercollectively referred to as a “linker,” is adapted for connecting thefolate analog to another molecule in other embodiments of the invention.A divalent linker L is present in the folate analog of formula I whenthe integer n is equal to 1. Such linkers are known in the art and areoften used to “associate” one chemical entity to another. As usedherein, the term “association” refers to any manner of coexistence oftwo or more molecules, such as complexation, chelation, ion-pairing,covalant bonding, and the like, such that for a time sufficient toadminister the associated molecules, the associated molecules may beinterpreted as a single entity.

The linker may create either a permanent or a semipermanent (i.e.,labile) linkage. The inclusion of a semipermanent linkage is especiallyadvantageous for applications in which cellular uptake of the drug isdesired. The ability to form a bioactive conjugate utilizing a linkageother than a peptide linkage (e.g., the glutamyl linkage of typicalfolate conjugates) provides an important degree of chemical flexibilityfor the linkage of the pteroic acid moiety to the drug payload. Thecapacity of a target cell for uptake of a folate-drug conjugate isexpected to be dramatically increased when a linkage is selected thatpromotes drug release from the conjugate by exploiting known endosomalhydrolytic or reductive mechanisms (i.e., molecular separation betweenthe drug payload and the ligand).

A preferred embodiment of the ligand-agent conjugate of the inventiontherefore includes a linker whereby a cell targeting ligand (i.e., thenon-peptide folic acid analog) is chemically coupled to a drug moleculevia a linker that is designed to be metabolized within the endosomalmilieu. Following extracellular receptor binding and endocytic entry ofthe drug conjugate, endosome factors are expected to hydrolytically orreductively cleave the linker moiety of the conjugate, therebyfacilitating release of the drug from the ligand. This process isdepicted below:

${FRBL} - X - {D\mspace{14mu}\overset{{endosome}\mspace{14mu}{milieu}}{\longrightarrow}\mspace{14mu}\left\lbrack {{FRBL} - {X'}} \right\rbrack} + D$wherein the abbreviations are as follows: FRBL, folate receptor bindingligand; X, endosome-cleavable linker; D, drug moiety; X′, linkerfragment.

A preferred semi-permanent linker thus includes a functionality, such asa disulfide, ester, other hydrolyzable group, and the like, that allowsseparation of the ligand and the agent once the conjugate has reachedthe treatment site.

Semi-permanent linkers preferably depend upon endogenous mechanisms ofcleavage, and include metabolically labile linkers, such as anucleotide, amide, ester, and the like subject to cleavage bypeptidases, esterases, phosphodiesterases, reductases, and the like,which provides a stable ligand-agent conjugate prior to delivery butallows cleavage upon reaching the target or treatment site. Preferredlinkers used to produce these drug conjugates are biologically labile(pH sensitive, redox sensitive, enzymatically sensitive) such that theligand-receptor complex can be separated from the macromolecule“payload” in a predetermined manner (e.g., following endocytosis). Theinclusion of a metabolically labile function is advantageously chosen inan end-use dependent manner such that following the binding of theconjugate, or additionally subsequent uptake of the conjugate asdescribed below, the metabolically labile association may be cleavedthus releasing the agent from the ligand, either locally(extracellularly) in the case of binding of the conjugate to the cellsurface, or intracellularly, as in the case of post-uptake by the cell.

The divalent linker L comprises a linear or branched chain comprising aplurality of linking groups L₁, L₂, . . . , L_(m), wherein “m” is aninteger from 0 to about 50. Preferably, m is selected such the number ofatoms in the linear backbone of linker L is at least about 1, morepreferably at least about 3, most preferably at least about 6; and atmost about 100, more preferably at most about 50, and most preferably atmost about 20.

Each linking group “L_(m)” is also a divalent moiety composed of atomsselected from the group consisting of carbon, nitrogen, oxygen, andsulfur, providing that an oxygen atom is not adjacent to another oxygenor sulfur atom, except when the sulfur atom is oxidized, as in —S(O)₂—.Each individual linking unit “L_(m)” can be the same or different and isthus independently selected from the group of divalent radicals.Illustrative divalent radicals are —CR^(6″)R^(7″)—,—(R^(6″))C═C(R^(7″))—, —CC—, —C(O)—, —O—, —S—, —SO₂—, —N(R^(3″))—,—(R^(6″))C═N—, —C(S)—, —P(O)(OR^(3″))—, —P(O)(OR^(3″))O—, and the like.

R^(3″) is a group suitable for nitrogen or oxygen attachment, such ashydrogen, C₁-C₄ alkyl, C₂-C₄ alkenyl, C₃-C₈ cycloalkyl, aryl, C₁-C₄alkanoyl, acryloyl, and the like. R^(3″) attached to nitrogen may alsobe hydroxy, C₁-C₄ alkoxy, amino, monoalkylamino, or dialkylamino. It isappreciated that R^(3″) may be selected independently for each linkinggroup L_(m).

R^(6″) and R^(7″) are each independently selected from groups suitablefor carbon attachment such as hydrogen, C₁-C₄ alkyl, C₂-C₄ alkenyl,hydroxy, halo, C₁-C₄ alkoxy, C₃-C₈ cycloalkyl, aryl, C₁-C₄ alkanoyl,acryloyl, and the like. In addition, R^(6″) and R^(7″) are selectedindependently for each linking group L_(m).

The linker L may also possess one or more cyclic regions, wherein asubset of the linking groups “L_(m)” form one or more rings, including,but not limited to divalent cycloalkyl, such as cyclopent-1,3-diyl,cyclohex-1,1-diyl, and the like; divalent heterocyclyl, such aspyrrolidin-1,3-diyl, piperidin-2,2-diyl; and divalent aromatic groups,such as 1,3-phenylene, pyrrol-1,2-diyl, and the like.

Illustrative linkers L are polyalkylenes, polyalkylene glycols such aspolyethylene glycol (PEG), N-(2-hydroxypropyl)methacrylamide (HPMA), andthe like. Other examples of such linkers may be found in U.S. Pat. Nos.6,207,157, 6,184,042, 6,177,404, 6,171,859, and 6,171,614, thedisclosures of which are incorporated herein by reference. The inventionis not intended to be limited by the nature or length of the linker L.

The term “alkenyl” as used herein refers to a linear or branched chainof carbon atoms, such as ethenyl, propenyl, 2-methylethenyl, and thelike.

The term “cycloalkyl” as used herein refers to a cyclic chain of carbonatoms, such as cyclopropyl, cyclopentyl, cyclohexyl, and the like.

The term “aryl” as used herein refers to an aromatic moiety, such asphenyl, pyridinyl, pyrimidinyl, and the like. The aryl group isoptionally substituted with from 1 to 3 substituents, such as with halo,alkyl, alkoxy, as defined above, and the like.

The term “acryloyl” as used herein refers to aryl, as defined above,substituted with a carbonyl group, such as benzoyl, picolinyl, and thelike.

The term “polyalkylene” as used herein refers to polymers of alkenes,such as polyethylene, polypropylene, and the like.

Synthesis of non-peptide folic acid analogs may be accomplished bymethods known to the skilled artisan. In addition, the optionalincorporation of a linker may also be accomplished by methods known tothe skilled artisan.

The present invention also provides a ligand-agent conjugate capable ofbinding to a folate recognition site, comprising a diagnostic ortherapeutic agent in association with a non-peptide folic acid analog ofgeneral formula II:

where

X, Y, U, V, W, T, A¹, A², R¹, R⁶, R⁷, L, n, p, r and s are as definedabove;

q is an integer ≧1; and,

D is a diagnostic agent or a therapeutic agent.

One embodiment of the invention is a ligand-agent conjugate capable ofbinding to a folate recognition site, such as a folate binding protein,folate receptor, and the like. Compounds illustrative of this embodimentare selected from the general formula II, where the integer q is equalto 1. Such analogs may operate as a means for targeting of and deliveryto cells or tissues expressing folate recognition sites. The compoundsof formula II may optionally include a linker L, where L is as definedabove and where the integer n is equal to 1.

Another embodiment of the present invention is a ligand-agent conjugatecapable of binding to a folate recognition site with high affinity,comprising a diagnostic or therapeutic agent in association with aplurality of non-peptide folic acid analogs of general formula II, wherethe integer q is 2 or greater. Similarly, such ligand-agent conjugatesmay optionally comprise a plurality of ligands each possessing a linkerL, where L is as defined above and where the integer n is equal to 1.Such conjugates possessing a plurality of folate analogs in associationwith the diagnostic or therapeutic agent may advantageously enhancerecognition of the conjugate by the recognition site.

The diagnostic or therapeutic agent D can be linked to the ligand at(L)_(n) by any type of molecular interaction including a covalent bond,and ionic bond or association, hydrogen bonding or other type ofcomplexation to form the ligand-agent conjugate.

Synthesis of ligand-agent conjugates may be accomplished by methodsknown to the skilled artisan depending upon the nature of theassociation of the ligand and the agent.

Virtually any type of molecule (small molecular weight chemotherapeutic,peptide, protein, oligosaccharide, antisense oligonucleotide, plasmid,ribozyme, artificial chromosome, micelle, liposome, etc.) can be moreefficiently delivered into cells using this technology.

Diagnostic agents useful in the present invention include compoundscapable of labeling a cell or tissue with a contrast agent for thegeneration or modulation of signal intensity in biomedical imaging. Suchcontrast agents may be used for imaging such cells and tissues usingtechniques such as Magnetic Resonance Imaging (MRI), radio-imaging,radio-diagnosis, and the like. Such labeling of the cell or tissue isillustratively accomplished by incorporation of superparamagnetic,paramagnetic, ferrimagnetic, or ferromagnetic metals, radioactivegamma-emitting, positron-emitting or photon-emitting metals,radionuclides, other radioactive elements such as certain halogenisotopes (radiohalogens), and the like, in the agent. The diagnosticagent may be a chelating agent capable of binding such metals describedabove, or a radio-pharmaceutical possessing an organic fragment, such asan aromatic ring, possessing a radiohalogen. Such chelating agents areknown to the skilled artisan.

Metals useful in the invention employing such chelating agents for MRIinclude certain ions of chromium, manganese, iron, cobalt, nickel,copper, praseodymium, neodymium, samarium, gadolinium, terbium,dysprosium, holmium, erbium, ytterbium, and the like, such as Cr(III),Mn(II), Fe(II), Fe(III), and Ni(II). Metals useful in the inventionemploying such chelating agents for radio-imaging include certainisotopes of gallium, indium, copper, technetium, rhenium, and the like,such as ^(99m)Tc, ⁵¹Cr, ⁶⁷Ga, ⁶⁸Ga, ¹⁰³Ru, ²¹¹Bi, ⁶⁴Cu and ¹¹¹In.Radiobalogens useful in the invention employing radio-pharmaceuticalsinclude certain isotopes of fluorine, iodine, astatine, and the like,such as ¹⁸F, ¹²³I, and ¹³¹I.

Visualization techniques suitable for radioimaging are known in the art,such as positron emission tomography (PET), planar or SPECT imaging,gamma cameras, scintillation, and the like.

Therapeutic agents useful in the present invention include compoundscapable of modifying, modulating, or otherwise altering certain cellularor tissue functions. Therapies include elimination of certain pathogeniccell populations or pathogenic tissues, enhancing beneficial functionsin host cells or host tissues, protecting host cells or host tissuesfrom non-selective treatment, and the like.

One embodiment of the invention is a ligand-therapeutic agent conjugatewherein the therapeutic agent targets pathogenic cells or tissues, suchas tumors, bacteria, and the like. Such therapeutic agents includechemotherapeutic agents, antimicrobial agents, or other cytotoxic agentsassociated with the targeting ligand. Such cytotoxic agents, may lead tothe destruction of the pathogenic cell or tissue. The therapeutic agentmay be a radiotherapeutic agent. These agents, like the relateddiagnostic agents above, may possess a chelating functionality capableof sequestering a radionuclide, such as a radioactive metal or aradioactive alpha or beta-emitting metal suitable for nuclear medicine,or alternatively a suitable functionality bearing a radiohalogen, suchas an aryl group. In this context however, the metal or halogen is usedfor radiotherapy rather than for radiodiagnosis. Metals appropriate forsuch radiotherapeutic agents are known in the art, including certainisotopes of gadolinium, technetium, chromium, gallium, indium,ytterbium, lanthanum, yttrium, samarium, holmium, dysprosium, copper,ruthenium, rhenium, lead, bismuth, and the like, such as ¹⁵⁷Gd, ⁶⁴Cu,⁶⁷Cu, ¹⁸⁶Re, ¹⁸⁸Re, ⁹⁰Y, ¹¹¹In, and ¹⁷⁷Lu. Radiohalogens are also usefulin the invention for radiotherapeutic agents, including certain isotopesof iodine, astatine, and the like, such as ¹²⁵I, ¹³¹I, and ²¹¹At. Inanother embodiment, the therapeutic may be a species suitable forneutron capture therapy, such as an organoborane moiety, comprising ¹⁰B.

Chemotherapeutic agents useful in the present invention include certainalkylating agents, such as busulfan, carboquone, chlomaphazine,lomustine, tubercidin, and the like, certain antimetabolites, such asfludarabine, doxifluridine, and the like, certain steroids and steroidanalogs, such as calusterone, testolactone, flutamide, tamoxifen,hexestrol, melengestrol, and the like, certain antiadrenals, such asmitolane, and the like, certain LH-RH analogs, such as buserelin,leuprolide, and the like, and certain anti-angiogenic agents.

Another embodiment of the invention is therapeutic ligand-agentconjugate that targets cells or tissue, such that a beneficial functionof the targeted cell or tissue is enhanced by the therapeutic agent,such as an inflammatory, pro-inflammatory, or anti-inflammatory agent,antibiotic, analgesic, antiviral agent, and the like. Still othertherapeutic agents useful in the present invention may protect atargeted cell or tissue from a subsequent non-selective treatmenttargeted to a different pathogenic cell or tissue, such as animmunosuppressant.

The present invention also provides a method for delivering a diagnosticor therapeutic agent to a targeted cell population. An effective amountof a ligand-agent conjugate comprising a diagnostic or therapeutic agentin association with a non-peptide folic acid analog of general formulaII, where the integer q is 1 or greater, is delivered to the targetedcell population. The targeted cells possess a folate receptor to whichthe ligand-agent conjugate binds. The ligand thus selectively targets acertain cell or tissue, by binding to the receptors or proteins presentin such cells or tissues that recognize the folic acid and folic acidanalogs. If desired, a plurality of non-peptide folate analog conjugatescan be administered.

In one embodiment of the method of the invention, the diagnostic ortherapeutic effect is achieved as a direct or indirect result of bindingof the ligand-agent conjugate to the folate receptor on the cell surface(i.e., “docking”). For example, in vivo biomedical imaging can befacilitated whether or not the diagnostic agent is internalized, and insome instances, for example in the case of a cytotoxic diagnostic agent,it is preferable that the diagnostic agent remain outside the cell. Asanother example, the therapeutic agent can include an immune stimulatingfactor such as an antigen, which is preferably retained on theextracellular surface on the cell.

In another embodiment of the method of the invention, the diagnostic ortherapeutic effect is achieved as a result of uptake or internalizationof the therapeutic or diagnostic agent via binding to the folatereceptor followed by internalization of the receptor-ligand complex. Itis appreciated that the method of the invention is suitable foreffecting uptake by cells or tissue of ligand-agent conjugates, wherethe agent is a molecule or compound that would otherwise exhibit pooruptake by the cell or tissue by active transport, diffusion, or otherpassive transport.

The targeted cell population can be endogenous to exogenous to thepatient. For example, it can be an endogenous population comprising asomatic or tumor cell population in a patient, a cancerous cellpopulation, an organ, tissue or bodily fluid, or a virus-infected cellpopulation. The ligand-agent conjugate can be delivered to a patientlocally or systemically. For example, the conjugate can be deliveredparenterally by intramuscular, intravenous or subcutaneous injection;likewise it can be formulated for oral administration.

An exogenous population of cells can include an ex vivo or in vitropopulation of cells. For example, the target cell population can be anex vivo population of cells such as bone marrow cells, stem cells, orcells of an organ or tissue that have been removed from the patient'sbody. The ex vivo cells are contacted with the ligand-agent conjugate ofthe invention and subsequently returned to the body of the patient. Genetherapy, for example, can be accomplished using a ligand-agent conjugateof the invention wherein the therapeutic agent is a nucleic acid.

Likewise the target population can be an in vitro population of cellssuch as a tissue or fluid sample or biopsy that has been removed from apatient for diagnostic purposes. The biological sample can be contactedwith the ligand-agent conjugate of the invention comprising a diagnosticagent for detection or characterization of the disease state of thepatient.

An exogenous population of cells can also include a population ofexogenous organisms such as bacteria, mycoplasma, yeast, or fungi,provided the organisms possess a receptor molecule that binds theligand-agent conjugate. See, e.g., Kumar et al., 1987, J. Biol. Chem.262(15):7171-9. The ligand-agent conjugate binds to the surface of thetumor cells or pathogenic organisms and “labels” or otherwise alters ormodifies cell or tissue function; it may or may not be internalized,depending on the intended application.

EXAMPLES

The following examples are illustrative of certain embodiments of theinvention. The examples, methods, and conditions presented therein arenot to be construed as limiting the scope nor the spirit of theinvention.

Example I Targeting the Tumor-Associated Folate Receptor with a¹¹¹In-DTPA Conjugate of Pteroic Acid

Objective. The present study was undertaken to evaluate the structuralrequirements for folate-receptor-targeting with low-molecular-weightradiometal chelates, specifically examining the role of the amino acidfragment of folic acid (pteroyl-glutamic acid) in mediatingfolate-receptor affinity.

Methods. The amide-linked conjugatepteroyl-NHCH₂CH₂OCH₂CH₂OCH₂CH₂NH-DTPA (CYK4-013), which lacks an aminoacid in the linker region, was prepared by a three-step procedure frompteroic acid, 2,2′-(ethylenedioxy)-bis(ethylamine), and t-Bu-protectedDTPA.

This conjugate (CYK4-013) was prepared as outlined in FIG. 2. CY3-064was obtained from pteroic acid (0.025 g; 0.080 mmol) and a large excessof 2,2′-(ethylenedioxy)-bis(ethylamine) (0.237 g; 1.6 mmol) using thecoupling reagents. Benzotriazole-1-yl-oxy-tris(pyrrolidino)phosphoniumhexafluorophosphate (PyBOP) (0.125 g; 0.24 mmol), N-hydroxybenzotriazole(HOBt) (0.037 g; 0.24 mmol), and N-methylmorpholine (Nmm) (0.049 g; 0.48mmol) in dry dimethylsulfoxide (DMSO) (0.8 mL) at room temperature for22 hours under nitrogen. The excess reagent and solvent DMSO wereremoved under vacuum and the resulting brown residue triturated withdiethylether, methanol, and water to produce 20 mg of CY3-064 as ayellow solid (57% estimated yield).

This yellow solid was then coupled with t-butyl-protected DTPA(synthesized as described by S. A. Chilefu, et al., J. Org. Chem.,2000,65:1562-1565) using the same coupling reagents (0.012 g CY3-064;0.071 g t-Bu-DTPA, 0.127 mmol; 0.0848 g PyBOP, 0.16 mmol; 0.0249 g HOBt,0.16 mmol; 0.0247 g Nmm, 0.24 mmol; 0.6 mL dry DMSO). During coupling,the solubility of CY3-064 was very poor in the DMSO solvent, and notimproved by addition of more Nmm.1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]-pyrimidine (MTBD)(0.0083 g; 0.0542 mmol) was added after stirring overnight at roomtemperature, but the solubility remained poor. After again stirringovernight, the DMSO and excess reagents were removed under high vacuumovernight.

The resulting brown residue was triturated first with diethylether andthen with methanol. The methanol suspension was centrifuged to producecrude CY3-078 as a yellow solid. This yellow solid was purified usingsemi-preparative HPLC (2×) on a C18 column (10×250 mm) to produce pureCY3-078. (HPLC solvent A=5% CH₃CN in 0.1% aqueous TFA; solvent B=10%water in 0.1% TFA in CH₃CN. Linear gradients established as: 5% B attime=zero ramping to 70% B at 30 minutes, then ramping to 100% B at 32minutes, and remaining 100% B to 40 minutes. Flow rate=2.35 mL/min). Thepeak with retention time of 30.5 minutes was collected. The purifiedCY3-078 was treated with 70% TFA/CH₂Cl₂ at 0° C. for 30 minutes, andstirred at room temperature for 5 hours, to remove the three t-Buprotecting groups. The resulting CYK4-013 product (6 mg) was isolated bytrituration with diethylether. Both CY3-078 and CYK4-013 exhibit theexpected parent ion peaks in their positive and negative ionelectrospray mass spectra. In particular, purified CYK4-013 exhibitedthe expected parent ion peaks in its positive and negative ionelectrospray mass spectra (m/e=818 and 816, respectively).

The ¹¹¹In complex of CYK4-013 was prepared from ¹¹¹In-chloride (1.2 mCi;44 MBq) and purified by reversed-phase HPLC. Specifically, the ¹¹¹Incomplex of CYK4-013 was prepared from ¹¹¹In-chloride via ligand exchangein acetate buffer. Briefly, 1.2 mCi no-carrier-added ¹¹¹In-chloride(Mallinckrodt, Inc., St. Louis) in 0.05 mL 0.05 N HCl was transferred toa small tube and 0.05 mL 0.1N ammonium acetate (pH 5.5), followed by0.02 mL 0.5N ammonium acetate (pH 7.4) was added, producing a solutionwith pH 7. The CYK4-013 ligand was weighed out and diluted in water, pHadjusted with 1N NaOH to pH 9-10. 10 μL of this ligand solutioncontaining 95 μg CYK4-013 was added to the ¹¹¹In-acetate solution (120μL) and mixed. The solution was protected from light and kept at roomtemperature.

The radiochemical purity of the resulting crude ¹¹¹In-CYK4-013 wasevaluated by radio-HPLC using a 4.6×250 mm Dynamax C18 column(Varian/Rainin) eluted with an aqueous NH₄OAc:acetonitrile gradient.HPLC conditions: Solvent A=56 mM NH₄OAc in water; Solvent B=CH₃CN. Flowrate=1 mL/min on 4.6×250 mm C18 reverse-phase column. Linear gradientconditions: 5% B at zero minutes, ramping to 25% B at 25 minutes, thenramping to 60% B at 27 minutes and 100% B at 30 minutes).

The major radioactive HPLC peak, eluting with a retention time of 14.1minutes, was collected. HPLC analysis of this isolated peak showed it toremain stable for at least 7 days at room temperature. HPLC conditions:Solvent A=56 mM NH₄OAc in water; Solvent B=CH₃CN. Flow rate=1 mL/min on4.6×250 mm C18 reverse-phase column. Linear gradient conditions: 5% B atzero minutes, ramping to 25% B at 25 minutes, then ramping to 60% B at27 minutes and 100% B at 30 minutes.

Radio-TLC of the HPLC-purified ¹¹¹In-CYK4-013 was performed using a C18plate eluted with 25% NH₄OAc; 75% acetonitrile. The results confirm theabsence of ¹¹¹In-species that irreversibly adsorb to C18 (i.e., there isno ¹¹¹In remaining at origin)

The HPLC-purified ¹¹¹In-CYK4-013 was removed from the HPLC solvents bysolid-phase extraction. A C18 Sep-Pak Light solid phase extractioncartridge (Millipore, Inc.) was conditioned by washing with ethanolfollowed by water. The ¹¹¹In-CYK4-013 was loaded onto the C18 Sep-Pakafter dilution with water to 5% acetonitrile, the Sep-Pak was washedwith 3 mL water, and the ¹¹¹In-CYK4-013 product was recovered byfractional elution with ethanol. The resulting ethanol solution of¹¹¹In-CYK4-013 was evaporated to dryness under a stream of N₂ at roomtemperature, and the ¹¹¹In-CYK4-013 was reconstituted in saline for usein a biodistribution study in mice.

Our primary animal model for evaluation of the biodistribution andpharmacokinetics of folate-receptor-targeted radiopharmaceuticals hasbeen athymic mice bearing subcutaneously implantedfolate-receptor-positive human KB cell tumors. Because normal rodentchow contains a high concentration of folic acid (6 mg/kg chow), themice used in these receptor targeting studies were maintained onfolate-free diet for 3 weeks to achieve serum folate concentrationsclose to the 4-6 μg/L (9-14 nM) range of normal human serum. After 3weeks on folate-free diet, mouse serum folate levels drop to 25±7 nMfrom the initial 720±260 nM serum folate level when the animals are fednormal rodent chow. This dietary intervention is believed to be areasonable manipulation of the animal model, since the mice would haveserum folate levels only slightly higher than the folate concentrationof normal human serum. Thus, in these mouse biodistribution studies theradiotracer is competing for tumor folate receptors with physiologicallyrelevant concentrations of endogenous unlabeled serum folate.

Thus, to demonstrate the ability of such conjugates to selectivelylocalize in folate-receptor-positive tissues, the biodistribution of¹¹¹In-CYK4-013 was determined following intravenous administration toathymic mice with subcutaneous folate-receptor-positive human KB celltumor xenografts. The resulting data are presented in Tables 1 and 2.The ¹¹¹In-CYK4-013 agent is found to selectively localize in thefolate-receptor-positive tumors (5.4±0.8 and 5.5±1.1 percent of theinjected ¹¹¹In dose per gram of tumor at 1 hour and 4 hourspost-injection, respectively) and to exhibit prolonged tumor retentionof the radiolabel (3.6±0.6 percent of the injected ¹¹¹In dose per gramof tumor still remaining at 24 hours post-injection). The tumorlocalization of the ¹¹¹In-radiolabel clearly appears to be mediated bythe cellular folate receptor, since the tumor uptake of radiotracerdrops precipitously (0.12±0.07 percent of the injected ¹¹¹In dose pergram at 4 hours) when ¹¹¹In-CYK4-013 is co-injected with an excess offolic acid, which will compete for folate receptor sites. Urinaryexcretion appears to be the primary whole-body clearance pathway for the¹¹¹In-CYK4-013. The substantial retention of ¹¹¹In in the kidneys isfully consistent with the binding of ¹¹¹In-CYK4-013 to tissue folatereceptors, since the renal proximal tubule is a known normal tissue siteof folate receptor expression. This interpretation is supported by theexpected and observed marked reduction in renal ¹¹¹In when¹¹¹In-CYK4-013 is co-administered with excess folic acid. The behaviorof the ¹¹¹In-CYK4-013 radiopharmaceutical in this animal model (Table 2)is very similar to that observed for ¹¹¹In-DTPA-Folate (Table 3).

Results. Biodistribution of ¹¹¹In-CYK4-013 is shown in Tables 1-3.Similar to ¹¹¹In-DTPA-Folate, ¹¹¹In-CYK4-013 selectively localized inthe folate-receptor-positive tumor xenografts, and afforded prolongedtumor retention of ¹¹¹In (5.4±0.8; 5.5±1.1; and 3.6±0.6% ID/g at 1 hour,4 hours, and 24 hours, respectively) (Table 2). The tumor localizationof the ¹¹¹In-radiolabel appears to be mediated by the cellular folatereceptor, since the tumor uptake dropped precipitously (0.12±0.07% ID/gat 4 hours) when ¹¹¹In-CYK4-013 was co-injected with an excess of folicacid (Table 2). Blockable binding was also observed in the kidneys,where the folate receptor occurs in the proximal tubules.

TABLE 1 Biodistribution of ¹¹¹In-CYK4-013 in KB Tumor-Bearing AthymicMice at Various Times Following Intravenous Administration Percentage ofInjected ¹¹¹In Dose Per Organ (Tissue) 1 Hour 4 Hours 4 Hours-Blocked**24 Hours Tumormass (g): 0.15 ± 0.10 0.080 ± 0.031 0.077 ± 0.010 0.104 ±0.097 Animalmass (g): 29.5 ± 1.2  28.6 ± 1.6  28.2 ± 1.1  28.6 ± 0.8 Animal Quantity & Gender: 3M 4M 4M 4M Blood:  0.29 ± 0.003 0.078 ± 0.0100.025 ± 0.015  0.055 ± 0.0002 Heart: 0.51 ± 0.09 0.41 ± 0.08 0.0023 ±0.0016 0.18 ± 0.06 Lungs: 0.69 ± 0.06 0.60 ± 0.01 0.0082 ± 0.0051 0.34 ±0.03 Liver & Gall Bladder: 6.8 ± 1.3 3.0 ± 1.0 0.074 ± 0.044 1.7 ± 0.9Spleen: 0.063 ± 0.016 0.049 ± 0.012 0.0049 ± 0.0029 0.055 ± 0.017 Kidney(one): 15.3 ± 1.2  20.2 ± 1.6  0.23 ± 0.15 26.8 ± 2.7  Stomach,Intestines & Contents: 5.8 ± 0.5 5.4 ± 0.8 4.6 ± 2.8 3.3 ± 0.6 Muscle:43.6 ± 4.1  33 ± 9  1.0 ± 0.7 23.1 ± 7.0  Tumor: 0.78 ± 0.47 0.46 ± 0.260.0094 ± 0.0061 0.42 ± 0.4  *Athymic mice (NuNu strain) withsubcutaneous tumors. Born Sep. 11, 2000. Arrived Oct. 10, 2000.Initiated folate free diet Oct. 10, 2000. Implanted on Oct. 20, 2000;0.25 × 10⁶ KB cells (passage 9) per animal subcutaneous in intrascapularregion. Study date: Nov. 2, 2000. Values shown represent the mean ±standard deviation. Blood was assumed to account for 5.5% of total bodymass. Muscle was assumed to account for 42% of the total body mass.**Folate receptors blocked by co-injection of folic acid dihydrate at adose of 4.1 ± 0.4 mg/kg.

TABLE 2 Biodistribution of ¹¹¹In-CYK4-013 in KB Tumor-Bearing AthymicMice at Various Times Following Intravenous Administration Percentage ofInjected ¹¹¹In Dose Per Gram (Tissue Wet Mass) 1 Hour 4 Hours 4Hours-Blocked** 24 Hours Tumormass (g): 0.15 ± 0.10 0.080 ± 0.031 0.077± 0.010 0.104 ± 0.097 Animalmass (g): 29.5 ± 1.2  28.6 ± 1.6  28.2 ±1.1  28.6 ± 0.8  Animal Quantity & Gender: 3M 4M 4M 4M Blood: 0.18 ±0.01 0.050 ± 0.009 0.016 ± 0.010 0.035 ± 0.002 Heart: 3.5 ± 0.4 2.9 ±0.9 0.015 ± 0.010 1.2 ± 0.5 Lungs: 1.5 ± 0.2 1.4 ± 0.3 0.041 ± 0.0270.72 ± 0.21 Liver & Gall Bladder: 4.3 ± 0.7 1.9 ± 0.6 0.051 ± 0.029 1.2± 0.6 Spleen: 0.31 ± 0.09 0.29 ± 0.07 0.028 ± 0.017 0.27 ± 0.08 Kidney(one): 61 ± 5  81 ± 7  0.90 ± 0.59 105 ± 7  Stomach, Intestines &Contents: 1.7 ± 0.2 1.5 ± 0.9 1.7 ± 1.1 1.2 ± 0.2 Muscle: 3.5 ± 0.5 2.8± 0.8 0.080 ± 0.057 1.9 ± 0.6 Tumor: 5.4 ± 0.8 5.6 ± 1.1 0.12 ± 0.07 3.6± 0.6 Tumor/blood 30 ± 5  111 ± 11  7.5 ± 2.4 105 ± 20  Tumor/kidney0.088 ± 0.013 0.069 ± 0.013 0.12 ± 0.03 0.035 ± 0.008 Tumor/liver 1.3 ±0.4 3.0 ± 0.5 2.2 ± 0.6 3.7 ± 1.4 Tumor/muscle 1.6 ± 0.4 2.1 ± 0.2 1.5 ±0.7 2.0 ± 0.6 *Athymic mice (NuNu strain) with subcutaneous tumors.Values shown represent the mean ± standard deviation. **Folate receptorsblocked by co-injection of folic acid dihydrate at a dose of 4.1 ± 0.4mg/kg.

TABLE 3 Biodistribution of ¹¹¹In-DTPA-Folate in Athymic Mice withSubcutaneous KB Cell Tumor Xenografts Percentage of Injectcd ¹¹¹In DosePer Gram (mean ± s.d.; n = 4) 4 Hours Post 1 Hour 4 Hours Injection PostInjection Post Injection BLOCKED Tumormass (g): 0.138 ± 0.051 0.202 ±0.083 0.193 ± 0.078 Animalmass (g): 24 ± 2  25 ± 1  24 ± 1  Folic AcidDose 0 0 495 ± 79  (μg/kg): Blood: 0.14 ± 0.03 0.064 ± 0.007 0.029 ±0.011 Heart: 2.3 ± 0.4 2.0 ± 0.3 0.022 ± 0.010 Lungs: 1.3 ± 0.1 1.1 ±0.3 0.065 ± 0.021 Liver & Gall 4.0 ± 1.5 2.2 ± 0.4 0.12 ± 0.03 Bladder:Spleen: 0.36 ± 0.03 0.35 ± 0.11 0.060 ± 0.021 Kidney: 90 ± 9  85 ± 12 2.3 ± 1.0* Stomach, 1.0 ± 0.2 1.0 ± 0.2 0.49 ± 0.20 Intestines &Contents: Muscle: 3.5 ± 0.8 2.9 ± 0.7 0.023 ± 0.013 Tumor: 5.3 ± 0.4 6.8± 1.2 0.16 ± 0.07 Tumor/blood 38 ± 7  106 ± 15  5.5 ± 0.8 Tumor/kidney0.060 ± 0.011 0.080 ± 0.012 0.050 ± 0.023 Tumor/liver 1.5 ± 0.5 3.3 ±1.1 1.4 ± 0.4 Tumor/muscle 1.6 ± 0.3 2.5 ± 0.9 8.4 ± 3.8 *n = 3 (While 4animals were studied, one gave an unusually high value for the kidneyuptake with no apparent underlying cause for the disparity with theother animals in this group. If that anomalous value is included, thisresult becomes 5.0 ± 5.5% ID/g, n = 4).

Conclusion. Tumor-selective drug targeting via the folate receptorremains feasible with pteroic acid conjugates lacking amino acidfragments, such as the glutamic acid moiety of folic acid.

Example II Synthesis of a DOTA Conjugate of Pteroic Acid

A pteroic acid conjugate linked to the tetraazamacrocyclic DOTAchelating ligand was prepared for radiolabeling with radiometals such as⁶⁴Cu²⁺ and ¹¹¹In³⁺. This conjugate (CY4-036) was prepared as shown inFIG. 3. The starting material for the synthesis was pteroic acid. Due tothe poor solubility of pteroic acid in organic solvent, pteroic acid wasprotected with 2-(trimethylsilyl)ethanol to produce intermediate CY4-033using literatural procedure (M. Nomura, et al., J. Org. Chem., 2000, 65,5016-5021) to increase its solubility in organic solvent before couplingto the DOTA derivative.

DOTA was coupled to 2,2′-(ethylenedioxy)bis(ethyleneamine) using PyBOP,HOBt, and NMM as coupling reagents in DMF to produce CY4-032. Protectedform of pteroyl-linker-DOTA (CY4-034) was obtained through the couplingof CY4-032 and CY4-033 in DMSO using MTBD as a base, and then purifiedvia flash chromatography eluted with gradient MeOH/CHCl₃.

All the protecting groups (three t-butyl groups on carboxylic acids andone 2-(trimethylsilyl)ethyloxycarbonyl on nitrogen) were removed by thetreatment with 70% TFA/CH2Cl2 to produce CY4-036.

Synthesis of CY4-032. To a solution of DOTA-tri-t-butyl ester (0.050 g,0.087 mmol), PyBOP (0.136 g, 0.261 mmol), and HOBt (0.052 g, 0.339 mmol)in DMF (0.9 mL) was added NMM (0.035 g, 0.348 mmol) under N₂. The clearsolution was stirred at room temperature (i.e., about 25° C.) for 10minutes followed by the addition of2,2′-(ethylenedioxy)bis(ethyleneamine) (0.065 g, 0.436 mmol) under N₂.After stirring at room temperature for 17 h, the solution wasconcentrated under high vacuum to remove DMF and excess reagents. Theoily residue was triturated with Et₂O (3×5 mL). After the removal ofEt₂O, EtOAc (3 mL) was added followed by 2 mL of water. After stirringfor 2 minutes, the EtOAc was separated from aqueous layer and then moreEtOAc (3 mL) was added for the extraction of product. The extraction wasrepeated one more time. All three EtOAc layers were combined,concentrated, and then dried under high vacuum to produce 0.153 g ofoily crude product. C₃₄H₆₆N₆O₉=702; Electrospray (+): M+H 703. Thiscrude material was used for the next coupling step without furtherpurification.

Synthesis of CY4-033. To a suspension of carbonyl diimidazole (CDI)(0.069 g, 0.425 mmol) and pteroic acid (0.028 g, 0.090 mmol) in DMSO(0.8 mL) was added triethylamine (0.032 g, 0.32 mmol) under N₂. Afterstirring at room temperature for 3.5 hours, 2-(trimethylsilyl)ethanol(0.076 g, 0.64 mmol) was added and stirred at room temperature for 5.5hours. The reaction mixture was concentrated under high vacuum overnightto remove DMSO and excess reagents. Yellow residue was produced andtriturated with Et₂O. A yellow solid (0.067 g) was produced as crudeproduct. This crude material was used for next reaction without furtherpurification.

Synthesis of CY4-034. To a solution of CY4-032 (0.153 g) and CY4-033(0.067 g) in DMSO (0.8 mL) was added MTBD (0.041 g, 0.27 mmol) under N₂.After stirring at room temperature for 21 hour, the reaction mixture wasdried under high vacuum to remove DMSO and excess reagents to produce0.157 g of yellow residue. This crude product was purified via flashchromatography eluted with gradient MeOH/CHCl₃ to produce 0.073 g ofpure CY4-034.

Synthesis of CY4-036. 70% TFA/CH₂Cl₂ (1 mL) was added to the purifiedCY4-034 (0.018 g) at room temperature. After stirring at roomtemperature for 4.5 hour, the reaction mixture was concentrated underreduced pressure and dried under high vacuum overnight to produce 19 mgof crude product. All three t-Butyl groups and the2-(trimethylsilyl)ethyloxycarbonyl protecting group were removed at thisstep.

⁶⁴Cu-complex of CY4-036. The ⁶⁴Cu complex of CY4-036 was prepared from⁶⁴Cu-chloride via ligand exchange in acetate buffer. Briefly, 2.88 mCiof no-carrier-added ⁶⁴Cu-chloride (Washington University, St. Louis,Mo.) in 0.005 mL of 0.01 N HCl was transferred to a small test tube andmixed with 0.010 mL of 0.5 M ammonium acetate (pH 7.4). The CY4-036ligand was weighed out and diluted in water and the pH adjusted with 1 NNaOH to pH 11-12. One μL of this ligand solution containing ˜125 μgCY4-036 was added to the ⁶⁴Cu-acetate solution and mixed. The pH of thesolution was adjusted to pH 8-9 with the addition of 1 μL of 1 N NaOH.The solution was protected from light and incubated at 65° C. for 30minutes.

The resulting crude ⁶⁴Cu-CY4-036 was diluted with water and injectedonto radio-HPLC using a 10×250 mm Dynamax C18 column (Varian/Rainin)eluted with an aqueous NH₄OAc:acetonitrile gradient. HPLC conditions:Solvent A=56 mM NH₄OAc in water; Solvent B=CH₃CN. Flow rate=2.35 mL/minon 10×250 mm C18 reverse-phase column. Linear gradient conditions: 5% Bat zero minutes, ramping to 25% B at 25 minutes, then ramping to 60% Bat 27 minutes and 100% B at 30 minutes. The major radioactive HPLC peak,eluting with a retention time of 20.9 minutes, was collected. Radio-TLCconfirmed the absence of ⁶⁴Cu(II)-acetate, which was independently shownto remain at the origin.

HPLC-purified ⁶⁴Cu-CY4-036 was removed from the HPLC solvents bysolid-phase extraction. A C18 Sep-Pak Light solid phase extractioncartridge (Millipore, Inc.) was conditioned by washing with ethanolfollowed by water. The HPLC-purified ⁶⁴Cu-CY4-036 was loaded onto theC18 Sep-Pak after dilution with water to 5% acetonitrile, the Sep-Pakwas washed with 20 mL water, and the ⁶⁴Cu-CY4-036 product recovered byfractional elution with ethanol. The resulting ethanol solution of⁶⁴Cu-CY4-036 was evaporated to dryness under a stream of N₂ at roomtemperature, and the ⁶⁴Cu-CY4-036 was reconstituted in water. AnalyticalHPLC confirmed the radiochemical purity, and stability, of the isolated⁶⁴Cu-CY4-036 product. HPLC conditions: Solvent A=53 mM NH₄OAc in water;Solvent B=CH₃CN. Flow rate=1 mL/min on 4.6×250 mm C18 reverse-phaseanalytical column. Linear gradient conditions: 5% B at zero minutes,ramping to 25% B at 25 minutes, then ramping to 60% B at 27 minutes and100% B at 30 minutes.

Example III Mandelic Acid Conjugate of Pteroic Acid

Biodegradation of an ester formed from pteroic acid and a substitutedderivative of mandelic acid is shown in FIGS. 4 and 5. FIG. 4illustrates bioreduction of the conjugate to release the drug ordrug-bearing moiety, while FIG. 5 illustrates acid hydrolysis of theconjugate to release the drug or drug-bearing moiety. Evidence from arecent article describing structure-activity relationships of the folatereceptor suggests that the proton on the nitrogen which forms part ofthe amide bond between pteroic acid and glutamic acid is not necessaryfor high-affinity binding (Westerhoff et al., 1995, MolecularPharmacology 48, 459-471). Accordingly, it is highly anticipated thatthe mandelate esters depicted in these schemes will bind with highaffinity to the folate receptor. An example is shown in Example IV.

Example IV Synthesis and Activity of (S)-α-carboxybenzoyl pteroate(ACBP) and N-pteroyl-2-amino-2-carboxymethylpyridine (Pte-AP)

Synthesis of ACBP. (S)-α-carboxybenzoyl pteroate (ACBP) is a mandelateester (see Example III). A solution of (S)-mandelic acid (28 mg, 0.187mmol) in 2 mL of anhydrous dimethylformamide was added via syringe to 30mg of a 60% dispersion of NaH in mineral oil (under argon). Afterstirring for 10 minutes at room temperature, solid pteroyl azide (64 mg,0.187 mmol) was added and the reaction was stirred for an additional 2hours. The reaction was quenched with a solution of 50 mg NH₄Cl in 60 mLof deionized water. The resulting solution was washed with hexanes (toremove the mineral oil) and then diethylether. The aqueous solution wassparged with argon while the flask was immerged in warm water toevaporate the residual diethylether. The solution was brought to pH 2.2by drop-wise addition of 1 N HCl, whereupon the product precipitated asa yellow-orange finely-divided solid. The solid was isolated bycentrifugation and washed twice with deionized water. The material wasdissolved in 6 mL of deionized water containing NH₄HCO₃ (16 mg, 0.2mmol). The resulting solution was filtered and then purified by HPLC:Novapak 19×300 mm prep column, gradient 0-40% B in 35 minutes; A=10 mMNH₄HCO₃, B=CH₃CN. R, about 16.5 minutes.

Relative binding affinity. To determine how well these compoundscompetes with 3H-folic acid for binding to the folate receptor(FR)-positive cell line, KB (available from the American Type CultureCollection, ATCC #CCL-17), a binding assay was conducted. The relativeaffinity of various folate derivatives was determined according to themethod described by Westerhoff et al. (Mol. Pharm., 1995, 48:459-471)with slight modification. Briefly, folate receptor-positive KB cellswere gently trypsinized in 0.25% trypsin in phosphate-buffered saline(PBS) at room temperature for 3 minutes and then diluted in folate-freeRPMI 1640 media (FFRPMI) (Gibco) supplemented with 10% heat-inactivatedfetal calf serum. Following a 5 minute 800×g spin and one PBS wash, thefinal cell pellet was suspended in FFRPMI (no serum). Cells wereincubated for 15 minutes on ice with 100 nM of ³H-folic acid in theabsence and presence of increasing concentrations of pteroate-containingtest articles. Samples were centrifuged at 10,000×g for 5 minutes, cellpellets were suspended in buffer, transferred to individual vialscontaining 5 mL of scintillation cocktail, and then counted forradioactivity. Negative control tubes contained only the ³H-folic acidin FFRPMI (no competitor). Positive control tubes contained a finalconcentration of 1 mM folic acid, and counts per minute (CPM) measuredin these samples (representing non-specific binding of label) weresubtracted from all samples. Relative affinities were defined as theinverse molar ratio of compound required to displace 50% of ³H-folicacid bound to folate receptor on KB cells, and the relative affinity offolic acid for the folate receptor was set to 1.

Results. The result of the binding assay are shown in FIG. 6. The esterACBP showed a relative binding activity of 0.46 compared to folic acid,and an EC50 of 204 nM compared to 93.4 nM for folic acid. The folateanalog containing an amide bond, Pte-AP, showed a relative bindingactivity of 0.48 compared to folic acid, and an EC50 of 193 nM comparedto 93.4 nM for folic acid.

Example V Pteroyl Hydrazide and Derivative

Synthesis of pteroyl hydrazide (Pte-hydrazide).N¹⁰-trifluoroacetylpteroic acid (40 mg, 0.098 mmol) andcarbonyldiimidazole (25 mg, 0.154 mmol) were dissolved in 2 mL ofdimethylformamide and stirred under argon at room temperature for 40minutes. Hydrazine (40 μL; 1.28 mmol) was added to the reaction vesselvia syringe. A precipitate immediately formed. Following 15 minutes ofstirring, several mL of deionized water were added, and the product wasisolated by centrifugation. No further purification was needed.

Synthesis of pteroylhydrazido-BTCA-DAS. The synthesis ofpteroylhydrazido BTCA-DAS is shown in FIG. 7. To a solution ofdiacetoxyscirpenol (DAS) (50 mg, 0.137 mmol) in 2.5 mL CH3CN was added30 mg (0.137 mmol) benzenetetracarboxylic dianhydride followed by 24 μLHünig's base (17.7 mg, 0.137 mmol, also known as DIPEA,diisopropylethylamine). The reaction mixture was stirred 1.5 hour underargon at room temperature. Some DAS remained, so an additional 6 mganhydride was added and stirring was continued an additional 1 hour and10 minutes. Pteroyl hydrazide (59 mg, 0.17 mmol) in 2.5 mL anhydrousdimethylsulfoxide (DMSO) was added, followed by an additional 24 μL(0.137 mmol) of Hünig's base. The reaction was stirred for 1 hour and 10minutes and precipitated by the addition of ethanol.

Binding activity. The binding activity of pteroyl hydrazide andpteroylhydrazido-BTCA-DAS was determined using the assay described inExample IV.

Results. The results of the binding assay are shown in FIG. 8. Pteroylhydrazide showed a relative binding activity of 0.74 compared to folicacid, and an EC50 of 94 nM compared to 70 nM for folic acid.Pteroylhydrazido-BTCA-DAS showed a relative binding activity of 0.60compared to folic acid, and an EC50 of 116 nM compared to 70 nM forfolic acid.

The complete disclosures of all patents, patent applications includingprovisional patent applications, and publications, and electronicallyavailable material (e.g., GenBank amino acid and nucleotide sequencesubmissions) cited herein are incorporated by reference. The foregoingdetailed description and examples have been provided for clarity ofunderstanding only. No unnecessary limitations are to be understoodtherefrom. The invention is not limited to the exact details shown anddescribed; many variations will be apparent to one skilled in the artand are intended to be included within the invention defined by theclaims.

1. A ligand-agent conjugate having the formula

wherein X and Y are each independently selected from the groupconsisting of halo, R², OR², SR³, and NR⁴R⁵; U, V, and W representdivalent moieties wherein U and W are —N═ or —N(R^(4″))— and V is—(R^(6′))C═ or —(R^(6′))C—(R^(7′))—; Q is selected from the groupconsisting of C and CH; T is selected from the group consisting of S, O,N and —C═C— such that the ring structure of which T is a member isaromatic; A¹ and A² are each independently selected from the groupconsisting of —C(S)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z), —O—C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, —C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur provided that A² does not represent—C(O)NH—; R¹ is selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″),R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂alkenyl, C₁-C₁₂ alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂alkylamino)carbonyl; R⁶ and R⁷ are each independently selected from thegroup consisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; orR⁶ and R⁷ are taken together to form O═; R^(6′) and R^(7′) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or R⁶ and R⁷ are taken together to formO═; L is a divalent linker; n, p, r and s are each independently either0 or 1 provided when n=1, then r=1; q is an integer ≧1; D is diagnosticagent or a therapeutic agent; and provided that the linker L does notinclude a naturally occurring amino acid covalently linked to A² at itsα-amino group through an amide bond.
 2. The ligand-agent conjugate ofclaim 1 comprising a metabolically labile linker L.
 3. The ligand-agentconjugate of claim 2 wherein the metabolically labile linker L ishydrolytically or reductively cleaved in the cell to release thediagnostic or therapeutic agent D.
 4. A pharmaceutical compositioncomprising the ligand agent conjugate of claim 1 and at least onecomponent selected from the group consisting of a pharmaceuticallyacceptable carrier, excipient, or diluent.
 5. A method for delivering adiagnostic agent or a therapeutic agent to a target cell populationcomprising a folate receptor, the method comprising: providing aligand-agent conjugate having the formula

wherein X and Y are each independently selected from the groupconsisting of halo, R², OR², SR³, and NR⁴R⁵; U, V, and W representdivalent moieties wherein U and W are —N═ or —N(R^(4″))— and V is—(R^(6′))C═ or —(R^(6′))C(R^(7′))—; Q is selected from the groupconsisting of C and CH; T is selected from the group consisting of S, O,N and —C═C— such that the ring structure of which T is a member isaromatic; A¹ and A² are each independently selected from the groupconsisting of —C(S)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z), —O—C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur provided that A² does not represent—C(O)NH—; R¹ is selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″),R⁶, and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂alkenyl, C₁-C₁₂ alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂alkylamino)carbonyl; R⁶ and R⁷ are each independently selected from thegroup consisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; orR⁶ and R⁷ are taken together to form O═; R^(6′) and R^(7′) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or R⁶ and R⁷ are taken together to formO═; L is a divalent linker; n, p, r and s are each independently either0 or 1 provided when n=1, then r=1; and; provided that the linker L doesnot include a naturally occurring amino acid covalently linked to A² atits α-amino group through an amide bond; q is an integer ≧1; and D isdiagnostic agent or a therapeutic agent; and contacting the target cellpopulation with an effective amount of ligand-agent conjugate to permitbinding of the ligand-conjugate to the folate receptor.
 6. The method ofclaim 5 wherein D is a diagnostic agent comprising a contrast agent foruse in medical imaging.
 7. The method of claim 5 wherein theligand-agent conjugate binds to the cell surface and is not internalizedby the cells of the cell population.
 8. The method of claim 5 whereinthe diagnostic or therapeutic agent D is internalized by the cells ofthe cell population.
 9. A ligand-agent conjugate having the formula

wherein X and Y are each independently selected from the groupconsisting of halo, R², OR², SR³, and NR⁴R⁵; U, V, and W representdivalent moieties wherein U and W are —N═ or —N(R^(4″))— and V is—(R^(6′))C═ or —(R^(6′))C—(R^(7′))—; Q is selected from the groupconsisting of C and CH; T is selected from the group consisting of S, O,N and —C═C— such that the ring structure of which T is a member isaromatic; A¹ and A² are each independently selected from the groupconsisting of —C(Z)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z), —O—C(Z)—N(R^(4″)), —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur provided that A² does not represent—C(O)NH—; R¹ is selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″),R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂alkenyl, C₁-C₁₂ alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂alkylamino)carbonyl; R⁶ and R⁷ are each independently selected from thegroup consisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; orR⁶ and R⁷ are taken together to form O═; R^(6′) and R^(7′) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or R⁶ and R⁷ are taken together to formO═; L is a divalent linker; n, p and r are each independently either 0or 1 provided when n=1, then r=1; and; q is an integer ≧1; and D isdiagnostic agent or a therapeutic agent provided that the linker L doesnot include a naturally occurring amino acid covalently linked to A² atits α-amino group through an amide bond.
 10. The ligand-agent conjugateof claim 9 comprising a metabolically labile linker L.
 11. Theligand-agent conjugate of claim 10 wherein the metabolically labilelinker L is hydrolytically or reductively cleaved in the cell to releasethe diagnostic or therapeutic agent D.
 12. The ligand-agent conjugate ofclaim 10 wherein the metabolically labile linker L comprises a disulfideor an ester.
 13. A pharmaceutical composition comprising theligand-agent conjugate of claim 9 and at least one component selectedfrom the group consisting of a pharmaceutically acceptable carrier,excipient, or diluent.
 14. A ligand-agent conjugate having the formula

wherein X and Y are each independently selected from the groupconsisting of halo, R², OR², SR³, and NR⁴R⁵; U, V, and W representdivalent moieties wherein U and W are —N═ or —N(R^(4″))— and V is—(R^(6′))C═ or —(R^(6′))C—(R^(7′))—; Q is selected from the groupconsisting of C and CH; T is selected from the group consisting of S, O,N and —C═C— such that the ring structure of which T is a member isaromatic; A¹ and A² are each independently selected from the groupconsisting of —C(Z)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z), —O—C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur provided that A² does not represent—C(O)NH—; R¹ is selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″),R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂alkenyl, C₁-C₁₂ alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂alkylamino)carbonyl; R⁶ and R⁷ are each independently selected from thegroup consisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; orR⁶ and R⁷ are taken together to form O═; R^(6′) and R^(7′) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or R⁶ and R⁷ are taken together to formO═; L is a metabolically labile divalent linker comprising a disulfideor an ester; n, p, r and s are each independently either 0 or 1 providedwhen n=1, then r=1; q is an integer ≧1; D is diagnostic agent or atherapeutic agent; and provided that the linker L does not include anaturally occurring amino acid covalently linked to A² at its α-aminogroup through an amide bond.
 15. The ligand-agent of claim 14 whereinthe metabolically labile linker L is hydrolytically or reductivelycleaved in the cell to release the diagnostic or therapeutic agent D.16. A ligand-agent conjugate having the formula

wherein X and Y are each independently selected from the groupconsisting of halo, R², OR², SR³, and NR⁴R⁵; U, V, and W representdivalent moieties wherein U and W are —N═ or —N(R^(4″))— and V is—(R^(6′))C═ or —(R^(6′))C(R^(7′))—; Q is selected from the groupconsisting of C and CH; T is selected from the group consisting of S, O,and N such that the ring structure of which T is a member is aromatic;A¹ and A² are each independently selected from the group consisting of—C(Z)—, —C(Z)O—, —OC(Z)—, —N(R^(4″))—, —C(Z)—N(R^(4″))—,—N(R^(4″))—C(Z), —O—C(Z)—N(R^(4″))—, —N(R^(4″))—C(Z)—O—,—N(R^(4″))—C(Z)—N(R^(5″))—, —O—, —S—, —S(O)—, —S(O)₂—, —N(R^(4″))S(O)₂—,—C(R^(6″))(R^(7″))—, —N(C≡CH)—, —N(CH₂—C≡CH)—, C₁-C₁₂ alkyl and C₁-C₁₂alkoxy; where Z is oxygen or sulfur provided that A² does not represent—C(O)NH—; R¹ is selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; R², R³, R⁴, R^(4′), R^(4″), R⁵, R^(5″),R⁶ and R⁷ are each independently selected from the group consisting ofhydrogen, halo, C₁-C₁₂ alkyl, C₁-C₁₂ alkoxy, C₁-C₁₂ alkanoyl, C₁-C₁₂alkenyl, C₁-C₁₂ alkynyl, (C₁-C₁₂ alkoxy)carbonyl, and (C₁-C₁₂alkylamino)carbonyl; R⁶ and R⁷ are each independently selected from thegroup consisting of hydrogen, halo, C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; orR⁶ and R⁷ are taken together to form O═; R^(6′) and R^(7′) are eachindependently selected from the group consisting of hydrogen, halo,C₁-C₁₂ alkyl, and C₁-C₁₂ alkoxy; or R⁶ and R⁷ are taken together to formO═; L is a divalent linker; n, p, r and s are each independently either0 or 1 provided when n=1, then r=1; q is an integer ≧1; D is diagnosticagent or a therapeutic agent; and provided that the linker L does notinclude a naturally occurring amino acid covalently linked to A² at itsα-amino group through an amide bond.
 17. A pharmaceutical compositioncomprising the ligand agent conjugate of claim 16 and at least onecomponent selected from the group consisting of a pharmaceuticallyacceptable carrier, excipient, or diluent.